1. Field of the Invention
The present invention relates to a color image forming apparatus and an image forming method, which can print a full-color image and a monochromatic image.
2. Description of the Related Art
In the prior-art, an electrophotographic full-color image is formed of four color toners comprising a black toner and three process color toners of cyan, magenta and yellow.
A color image forming apparatus is first described.
There is known a tandem-type color image forming apparatus wherein four-color image forming units (each comprising a photoconductor body, a charging device, an exposure device, a developing device and a transfer device) are arranged over a transfer medium (of direct type or indirect type) and a full-color image is formed by single passage of the transfer medium. There is also known a 4-rotation type color image forming apparatus wherein four-color developing devices and a single photoconductor body unit (comprising a photo-conductor body, a charging device, an exposure device and a transfer device) are provided and, in a case of forming a four-color image, a transfer medium (of direct type or indirect type) is rotated four times and four-color toner images are overlapped, thereby forming a full-color image. In short, color image forming apparatuses fall into two categories: tandem type and 4-rotation type.
In the case of the tandem type, a full-color image is formed by single passage of the transfer medium. On the other hand, in the case of the 4-rotation type, an approximately four times longer time is needed for image formation. The tandem type is more advantageous for high-speed full-color image formation.
A full-color (chromatic) toner, however, requires more transparency than a monochromatic (achromatic) toner in order to increase a color reproduction range. In order to obtain desired transparency, the full-color toner requires a more quantity of heat for fixation than the monochromatic toner. Hence, it is difficult to increase the printing speed of the full-color image forming apparatus up to a level of a dedicated monochromatic image forming apparatus.
When a monochromatic single-color image is to be formed, it is desirable to stop the operations of non-used color image forming units in order to prevent degradation of replaceable parts or consumable parts. Although this is possible in the structure of the tandem type, the mechanism becomes complex and there is difficulty.
On the other hand, in the 4-rotation type, the speed for forming a full-color image is low, but it should suffice if only necessary color developing units are put in contact with the photoconductor body. Thus, when a monochromatic single-color image is formed by the 4-rotation type, a printing speed that is substantially equal to that of the tandem type can be obtained, and the non-used color developing units may be stopped. Thus, no special mechanism for preventing degradation is needed. Furthermore, since only one photoconductor unit is used, the size of the apparatus can be made smaller than in the tandem type.
As has been described above, the tandem type and 4-rotation type have advantages and disadvantages. It is difficult to meet all the requirements for the color image formation speed, prevention of degradation in consumable parts, and simple structure.
Next, cleaning is described.
With a cleaning device, a cleaning blade abrades a surface layer of the photoconductor body, leading to a decrease in life of the photoconductor body. A simultaneous development/cleaning process can increase the life of the photoconductor body by dispensing with the cleaning device. In this process, residual toner after primary transfer is recovered from a development area into each developing device. This process is feasible in the tandem type since photoconductor bodies are provided for the respective colors, but it is substantially unfeasible in the 4-rotation type.
Next, a transfer method is described.
Transfer methods fall into two categories: a direct transfer method and an indirect transfer method. In the direct transfer method, a photo-conductor body and a transfer medium, such as paper, are put in direct contact, and a toner image is transferred. In the indirect transfer method, a toner image is once transferred from a photoconductor body to an intermediate transfer member, and then the toner image is secondarily transferred from the intermediate transfer member to a transfer medium such as paper. Since the toner image is gradually degraded as it passes through process steps, the direct transfer method, in which the toner image is only once transferred from the photoconductor body to the transfer medium, is advantageous in consideration of specks of toner.
Since 100% of toner is not transferred, loss of toner due to post-transfer residual toner is minimized if the number of times of transfer is one.
The conditions of the fed transfer medium (e.g. thickness of paper, surface smoothness, moisture ratio due to environmental conditions, etc.) are variable. Thus, in the direct transfer method, it is difficult to keep constant the transfer potential conditions at four direct transfer locations. In the direct transfer method, the color reproduction varies if the transfer efficiency slightly varies. Consequently, it is difficult to obtain stable color reproducibility.
On the other hand, in the indirect transfer method, the possibility of degradation in image quality due to dispersion of toner is higher than in the direct transfer method, and the loss of toner due to occurrence of post-transfer residual toner may possibly be greater. However, four color toners are overlapped on the intermediate transfer member that is kept in the fixed environmental condition within the apparatus. It is thus easier to maintain the image quality, compared to the case where toners are overlapped directly on the final transfer medium. Furthermore, the indirect transfer method requires only one-time transfer to the final transfer medium that is unstable in terms of conditions, so the effect due to a variation in transfer conditions such as environment can be minimized. Therefore, such an advantage is obtained that the color reproducibility of color images can easily be made uniform. Besides, the degree of freedom is high in the design of the transfer path for the final transfer medium.
As has been described above, both the direct transfer method and indirect transfer method have advantages and disadvantages in terms of the image quality and toner consumption efficiency.
Jpn. Pat. Appln. KOKAI Publication No. 03-214174 discloses a technique wherein in a color print mode, a toner image is indirectly transferred to a transfer medium via an intermediate transfer member, and in a monochromatic print mode, a toner image is directly transferred to a transfer medium. In this method, four color developing devices are arranged around a single photoconductor body, and the photoconductor body is rotated by the number of times, which corresponds to the number of colors, thereby forming a color image. In this method, there is a large difference in printing speed between a full-color image and a monochromatic image, and the customers' needs cannot be satisfied. At the time of full-color image formation, black toner, as well as chromatic toners, is subjected to an intermediate transfer process step. Consequently, the sharpness of a black image in a full-color image cannot be expected.
Jpn. Pat. Appln. KOKAI Publication No. 09-120190 discloses a color recording apparatus having a first mode, in which toner on a photoconductor body is directly transferred, and a second mode, in which the toner is intermediately transferred. In the second mode, the intermediate transfer belt rotates in a first direction for transfer from the photoconductor body, and in a direction opposite to the first direction, for transfer from the intermediate transfer belt to a transfer medium. However, to change the direction of rotation of the intermediate transfer member according to the modes requires a complex mechanism and is not desirable. In addition, reverse rotation in the intermediate transfer method makes it necessary to reverse image data, and this disadvantageously leads to a complex process. In this method, too, in the case of a full-color image, black toner is also transferred to a transfer medium via intermediate transfer. Consequently, the sharpness of a black image cannot be achieved.
Jpn. Pat. Appln. KOKAI Publication No. 2001-75331 discloses a technique wherein post-transfer residual toner on an intermediate transfer member is re-charged with an opposite polarity by a re-charging device, and transferred at a transfer position of a black image carrying body that is located at the most upstream part of the intermediate transfer member. In this invention, a black image forming unit is disposed at the most upstream part of the intermediate transfer member. On the downstream side of the black image forming unit, cyan, magenta and yellow image forming units are arranged. In the order of arrangement of image forming units, toners are overlapped on the intermediate transfer member and are transferred at a time on a final transfer medium in a secondary transfer section. In this case, in the image part on which a plurality of color toners overlap on the intermediate transfer member, a black toner that is far from the transfer medium is least easily transferred, and it is highly possible that residual toner occurs after transfer. Consequently, a black character on a color background, for instance, is not clearly transferred, and a line width may become inadequate due to low density. Moreover, the sharpness of an edge part would disadvantageously be lost.